Various types of metallic materials have been used in implanted medical devices in the past. Type 316L or a 316LVM stainless steel, cobalt-chromium alloys, commercially pure titanium, and titanium alloys are typical metals used for implantable devices. The environment and method of implantation dictates the use of certain raw materials with specific biocompatibility and material properties. These materials typically possess the necessary physical properties such as tensile strength, fatigue resistance, elastic recoil and yield strength for specific applications.
It is often desirable to form these metallic materials into complex shapes (including diametrically expandable shapes) such as artificial heart valves, stents, and filters. These types of applications would typically require a metallic material with strength properties close to that of 316L or a 316LVM stainless steel as well as an elastic recoil similar to 316L or a 316LVM. There are often applications that require that these complex shapes be expanded in size (e.g., via a balloon) to conform or comply with certain geometry, be that anatomical or device-driven geometry. In these applications, the metallic material selected would have a relatively low yield strength to allow ease of expansion. The intended environment of implantation of some of these devices (e.g., coronary stents) typically requires a metallic material with a relatively high strength.
Device geometry, method of delivery and environment often force the choice of a metallic material that compromises in one of the four important physical property areas: tensile strength, fatigue resistance, elastic recoil or yield strength. For these reasons, the choice of a metallic material for a particular application is often challenging and compromising.
In relation to other advanced high-strength steels, multiple phase steels (i.e., multi-phase steels) exhibit better ductility at a given strength level. In an example of one multiple phase steel, dual phase steel, the enhanced formability stems from the combination of ferrite and martensite phases present in the raw material. Dual phase steel has a high work hardening rate that enables it to behave in a stable manner during a stamping or forming process. Dual phase steel may be purchased from a supplier such as AK Steel (West Chester, Ohio).
In another example of multiple phase steels, TRIP (Transformation Induced Plasticity) steel, enhanced formability comes from the transformation of retained austenite (ductile, high temperature phase of iron) to martensite (tough, non-equilibrium phase) during plastic deformation. Enhanced formability also stems from a high work hardening rate, which enables the metal to behave in a stable way during a stamping or forming process. Because of this increased formability, TRIP steel may be used to produce more complex shapes than other high strength steels. TRIP steel may be purchased from suppliers such as US Steel (Pittsburgh, Pa.) or ArcelorMittal (Brazil).
TRIP steel containing 4% Mo has been evaluated against type 316L or a 316LVM stainless steel and cast Vitallium alloy as a potential material for use as an implantable material for orthopedic applications. Results from in vivo evaluation of TRIP steel versus 316L stainless steel in these applications showed that TRIP steel was susceptible to stress-corrosion cracking and much more susceptible to crevice corrosion.